Method of induction hardening for improving fatigue strength of boundary of heated zone

ABSTRACT

This invention provides a method for reducing the residual tensile stresses generated at the boundaries of the hardened zone of a locally case-hardened steel object by inducing a quantity of eddy currents in the boundaries which gradually decreases in a direction outwardly away from the hardened zone and toward nonhardened zones.

United States Patent Tomita et al.

[151 3,658,605 [451 Apr. 25, 1972 [54] METHOD OF INDUCTION HARDENING FOR IMPROVING FATIGUE STRENGTH OF BOUNDARY OF HEATED ZONE [72] Inventors: Katsunobu Tomlta; Kentaro lshll, both of Tokyo; Yoshito Tanaka, Nara; Takao Salto, Nishinomiya, all of Japan [731 Assignees: Japanese National Railways, Tokyo,

Japan; Sumltomo Metal Industries Limited, Osaka City, Japan [22] Filed: Sept. 17, 1969 [21] App1.No.: 858,811

[30] Foreign Application Priority Data Sept. 18, 1968 Japan ..43/67741 [52] U.S.Cl ..l48/l54 [51] Int. Cl. ..C21d 1/10, C2ld 1/66 [58] Field of Search ..l48/148, 149, 150, 154

[56] References Cited UNITED STATES PATENTS A 2,303,408 12/1942 Soderholm 148/150 2,845,377 7/1958 Seulen et a1. 148/150 2,249,909 7/1941 Pisarev ..148/1 50 2,292,209 8/1942 Denneen et a1... ..148/148 2,430,005 11/1947 Denneen et a1 ..l48/149 2,549,930 4/1951 1 Riegel etal ..148/148 Primary Examiner-1... Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Watson, Cole, Grindle & Watson [57] ABSTRACT This invention provides a method for reducing the residual tensile stresses generated at the boundaries of the hardened zone of a locally case-hardened steel object by inducing a quantity of eddy currents in the boundaries which gradually decreases in a direction outwardly away from the hardened zone and toward non-hardened zones.

2 Claims, 4 Drawing Figures Patented April 25, 1972 I 2 Sheets-Shoot l FAQS/004A INVENTOR ATTORNEY Patented April 25, 1972 2 Sheets-Sheet 2 INVENTOR KM 7 Zii ATTORNEY METHOD OF INDUCTION HARDENING FOR IMPROVING FATIGUE STRENGTH OF BOUNDARY OF HEATED ZONE This invention relates to a method for producing a locally case-hardened steel object having improved fatigue strength characteristics at the boundaries of its hardened zone. Objects to which the method may be applied include, inter alia, straight or curved shafts for railway car axles, armature shafts, mill rolls and crank shafts.

Generally, steel objects are induction-hardened to improve their resistance to fatigue and wear. However, it is known that when steel objects are locally induction-hardened, the fatigue strength of such objects will be decreased at the boundaries of the hardened zone and therefore the susceptibility of the steel object to breakage will be correspondingly increased.

An object of the present invention is to provide a method for preventing the'reduction of the fatigue strength in locally case-hardened steel objects by reducing the residual tensile stresses generated at the boundaries of the hardened zone.

A feature of the method of the present invention is that the eddy currents induced in an object during induction-hardening are applied in a manner to establish a gradient which decreases gradually across the boundaries of the hardened zone so that heating temperature is gradually decreased across the boundaries and the distribution of residual stresses produced in the boundaries of the hardened zone is improved.

In applying the method of the present invention, the following arrangements can be utilized:

1. an arrangement of coils wherein the inside diameter, the axial spacing or the cross-sectional area of the coils gradually increases in an outward direction on each side of a conventional set of coils; (2) an arrangement of parallel coils wherein the current of each coil can be independently controlled; or (3) the material being hardened may be reciprocated repeatedly in the axial direction in a conventional stationary hardening apparatus.

Through the use of the method of the'invention, the eddy currents induced at the boundaries of a hardened zone may be gradually reduced in a direction outwardly from the hardened zone and toward the non-hardened zone and the temperature gradient across the boundaries of the hardened zone facilitates gradual temperature drops.

IN THE DRAWINGS:

FIG. 1 is a graph illustrating relationship between the position of a conventional induction coil and the distribution of residual stresses on the surface of a steel object hardened by a conventional local induction-hardening method;

FIG. 2 is a graph comparing the distribution of residual stresses on the surface of a shaft hardened by the conventional method and of a shaft hardened by the method of the present invention;

FIG. 3 is an elevational, longitudinal view, partly in cross section, showing an embodiment of the present invention; and

FIG. 4 is an elevational, longitudinal view, partly in cross section, showing another embodiment of the invention.

Before explaining the present invention, a conventional method shall be described with reference to FIG. 1 wherein the line Y represents the distribution of residual stresses in an axial direction along the surface of a conventionally hardened straight cylindrical shaft. In FIG. 1, X refers to an induction coil and Z refers to a hardened zone on the surface of the steel material (shown in dashed lines). The reference letters X, Y and Z in FIG. 1 are all shown as being disposed at corresponding axial positions. As is understood from FIG. 1, at locations within the zone 2, large residual compressive stresses are developed. However, at the areas adjacent the boundaries of the hardened zone, the character of the residual stresses is reversed and residual tensile stresses are developed. These residual tensile stresses are quickly reduced after passing a maximum. Thus, in the case of steel objects locally hardened by conventional induction-hardening, residual compressive stresses will'be developed inthe hardened zone itself. However, at. areas adjacent the boundaries of the hardened zone, residual tensile stresses will be developed causinga reduction in the fatigue strength of the object and an increase of its susceptibility to breakage.

The present invention is explained below with reference to the drawings. In FIG. 2, which is a graph plotted from the resultsof an X-ray'examination and measurement'of the residual stresses on the surface of ahardened object and plots the intensity of such stresses against distance along the axis of a locally induction-hardened shaft. The line A is of the case of theconventionally hardened shaft while lines B and C are of cases of shafts hardened by the method of thepresent invention.'The object hardened in-each case is a shaft of a carbon steel (AlSl 1038) containing 0.38 percent C and having a diameter of 190mm. The induction-hardening conditions in the case of the conventional method represented by'A in FIG. 2 were that the zone to be hardened was heated to a surface temperature of 830 C. fora heating time of seconds using a hollow copper'induction coil having an inside diameter of 225 millimeters. The coil had 6 windings in series and the cross-sectional dimensions of the coil itself were 20 millimeters deep by 12 millimeterswide. The hardened shaft was quenched by jetting water.

With regard to line B in FIG. 2, which represents the embodiment of the present invention shown in FIG. 3, the induction coil 1 has an inside diameter of 225 millimeters and has 6 windings in series, each having cross-sectional dimensions of 20 millimeters deep by 12 millimeters wide as in the conventional method. In FIG. 3, the induction coil 2 is made of copper and each winding has the same cross-sectional dimensions as the coil 1. The inside diameter of each winding of coil 2 is l0 millimeters larger than the inside diameter of the preceding winding. A coil 2 is added on each side of coil 1 as can be seen in FIG. 3..And the steel shaft was hardened under the same conditions of heating time and surface temperature as in the conventional method.

With regard to line C in FIG. 2, which represents the embodiment of the present invention shown in FIG. 4,the induction coil 3 is made of Cu and has the same cross-sectional dimensions as the coil 1 and the same inside diameter as the coil used in the conventional method. Coil 1 in FIG. 4 has a pitch such that adjacent windings are spaced with their centers disposed 20 millimeters apart. Coil 3 has two windings, and its pitch gradually increases such that the spacing between adjacent windings increases to 28 millimeters and then to 32 millimeters. A coil 3 is added on each side of ,a conventional coil 1. The object was hardened under the same conditions of heating time and surface hardening temperature as in the conventional method.

As is evident from FIG. 2, in the method of the present invention (lines B and C), as compared with the conventional method (line A), residual tensile stresses produced in the boundaries of the hardened zone are remarkably decreased, and therefore decreased fatigue strength at the boundaries of the hardened zone can be prevented.

The method of the present invention can also be worked by adding independently controllable, parallel heating coils on opposite sides of the induction coil. Also, the object being hardened may be repeatedly axially reciprocated in a stationary induction heating coil.

What is claimed is:

I. A method for producing a locally case hardened steel object having improved fatigue strength characteristics comprising:

locally case hardening a steel object by inducing eddy currents in the same at a zone thereon constituting less than the entirety of its outer surface to present a hardened zonebounded by at least one boundary area; and

preventing the development of excessive residual tensile stresses at said boundary area by simultaneously with said hardening inducing a decreased quantity of eddy currents in a portion of said boundary area directly adjacent the tion to said object in a manner to establish an eddy current profile across said portion of the area with the amount of eddy currents decreasing in a direction outwardly away from said zone. 

2. A method as set forth in claim 1 wherein said inducing a decreased quantity of eddy currents includes applying induction to said object in a manner to establish an eddy current profile across said portion of the area with the amount of eddy currents decreasing in a direction outwardly away from said zone. 